This invention relates to a projection exposure apparatus that exposes a pattern formed on a mask to transfer the pattern onto a photosensitive substrate.
In a manufacturing process for semiconductor devices or liquid crystal display devices, a projection exposure apparatus is conventionally used to illuminate a pattern formed on a photomask (or a reticle) with an illumination light and project an image of the pattern onto a photosensitive substrate. Typically, a semiconductor wafer or a glass plate coated with photosensitizer (e.g., photoresist) is used as the photosensitive substrate.
Currently, a step-and-repeat type projection exposure apparatus is widely used. With a step-and-repeat type projection exposure apparatus, a photosensitive substrate is loaded on a substrate stage, which is movable in two dimensions, and the shot areas arranged on the photosensitive substrates are successively exposed into the pattern image of the mask by shifting the photosensitive substrate in a stepwise manner by moving the substrate stage. A scanning-type projection exposure apparatus is also widely used, in which both the mask and the photosensitive substrate are scanned synchronously, thereby exposing the photosensitive substrate into the pattern image of the mask. Two types of scanning type projection exposure apparatus are known, a step-and-scan type apparatus, in which moving from one shot area to the next shot area is performed in a stepwise manner, and a collective-exposure type apparatus, in which the pattern of a large mask is transferred by scanning onto the entire surface of a single photosensitive substrate.
All of these projection exposure systems require highly accurate alignment in order to precisely superpose a pattern of the mask onto the pattern that has already been formed on the photosensitive substrate. The alignment is generally performed by detecting an alignment mark formed on the mask and/or photosensitive substrate using a detection system installed in the projection exposure apparatus.
U.S. patent application Ser. No. 418,260 (filed on Oct. 6, 1989) now U.S. Pat. No. 5,734,478, discloses an LSA (Laser Step Alignment) system that detects the position of an alignment mark formed by several columns of dots by illuminating laser light onto the dot sequence to obtain the light diffracted or scattered by the alignment mark. An FIA (Field Image Alignment) system uses a halogen lamp as a light source to illuminate the alignment mark with a light beam having a broad waveband width. The FIA system picks up the image of the alignment mark and processes the image data of the alignment mark. An LIA (Laser Interferometric Alignment) system illuminates a diffraction-grid alignment mark with laser beams from two different directions and picks up the interference signal of the diffracted light generated from the alignment mark, thereby detecting the position of the alignment mark. The LIA system includes a homodyne system, which is disclosed in U.S. Pat. No. 4,636,077 and uses two laser beams having the same frequency, and a heterodyne system, which is disclosed in U.S. Pat. No. 5,734,478 and uses two laser beams having slightly different frequencies.
Alignment methods are grouped into a TTL (Through-the-Lens) method, a TTR (Through-the-Reticle) method, and an off-axis method. In the TTL method, the position of the photosensitive substrate is measured through the projection optical system. In the TTR method, the positional relation between the reticle and the photosensitive substrate is detected through the reticle. In the latter case, the reticle is used as both a mask and a projection optical system. In the off-axis method, the position of the photosensitive substrate is directly measured without using the projection optical system.
In the LIA system, an alignment mark formed in the vicinity of the pattern field of the mask and an alignment mark formed in the vicinity of a shot area on the photosensitive substrate are simultaneously measured by the alignment system positioned above the mask to directly detect an offset (displacement) between the two marks. Based on the detected offset, the mask or the photosensitive substrate is finely adjusted so that the amount of the offset (displacement) becomes zero.
According to this method, illumination light whose waveband is insensitive to the photoresist layer is used as alignment light. For example, as shown in FIG. 9, a laser beam emitted by a semiconductor laser LD and having a wavelength of 690 nm is used. Such a semiconductor laser LD is driven by a driving circuit, which includes an auto output control (APC) circuit 52 to output constant level light. The alignment system installed above the mask guides the alignment light projected onto the mask surface and the alignment light projected onto the photosensitive substrate surface along the same axis from the semiconductor laser to an object lens of the alignment system. The alignment light exiting from the object lens illuminates a diffraction-grid alignment mark formed on the mask (hereinafter, referred to as a reticle mark) and a diffraction-grid alignment mark formed on the photosensitive substrate (hereinafter, referred to as a substrate mark). Taking the advantage of the fact that the two alignment light beams are substantially coaxial, the alignment system photoelectrically detects the light information from the reticle and the light information from the substrate at the same time.
FIG. 10 illustrates the relation between a reticle mark RM formed on the mask 1 and the alignment light. The reticle mark RM is usually formed on the bottom surface 1B of the mask 1, which lies on the opposite side of the surface 1A, which receives the incidence light. The alignment light fluxes fl and f2 coming from two directions are guided onto the reticle mark RM. The light flux f1 is reflected and diffracted by the reticle mark RM formed on the bottom surface 1B of the mask 1. Then, the 0-th order reflected light f1 returns to its normal direction. However, a fraction of the 0-th order diffracted light is internally reflected on the top surface 1A of the mask 1, and then again reflected and diffracted by the reticle mark RM. As a result, the flux f1 becomes multiplexed reflected/diffracted light, and returns in the same direction as the 0-th order diffracted light. (FIG. 10 does not illustrate the light that transmits the bottom surface 1B of the mask 1.) The regularly reflected light of the flux f1, which exists on the top surface 1A of the mask 1, interferes with the multiplexed reflected/diffracted light and the 0-th order diffracted light. Hence, the detection signals generated from the reticle mark RM interfere with each other as they reach the photoelectric detector. The same applies to the light flux f1, but f2 acts from the opposite direction.
In general, laser light used as alignment light is highly coherent, and depending on the kind of semiconductor laser used, the wavelength of the emitted light changes due to a mode hop phenomena, which induces undesirable changes in the interference conditions. As a consequence, the detection signals from the reticle mark are disrupted and the alignment accuracy (reproducibility) worsens.
The mode hop phenomena of a semiconductor laser is considered to be generated by changes in temperature, temporal changes, and the influence of the returning light (the light that is reflected on a plane perpendicular to the optical axis of the optical device comprising the optical system and that returns to the laser light source) and the like. As illustrated in FIG. 11, one or more monochromatic wavelengths xcex1, xcex2, xcex3, . . . generated by signal mode oscillation appears at random in a neighborhood of the central wavelength xcex0. These different wavelengths make the interference among the 0-th order diffracted light, the regularly reflected light, and the multiplexed reflected light unstable. As a result, the alignment between the mask and the photosensitive substrate becomes inaccurate, which reduces manufacturing yield.
In order to prevent the mode hop phenomenon, a temperature control method, which fixes the temperature of the semiconductor laser within a modestabilization range, is known. This method can temporarily stabilize the mode; however, in the long run, temporal mode hops occur even if the temperature remains constant. Moreover, depending on the condition of the returning light, the mode can become unstable regardless of the temperature. Hence, the temperature control method does not solve the fundamental problem.
The influences of the regularly reflected light, the multiplexed reflected/diffracted light and the 0-th order diffracted light, which arise on the top and bottom surfaces of the mask and reticle mark, has been explained above. Similar problems arise on the photosensitive substrate as well. A pattern layer formed on the photosensitive substrate surface and/or the top and bottom surfaces of the resist layer correspond to the top and bottom surfaces of the mask. Although explanation has been made using the LIA system as an example, all the alignment systems that utilize laser light scattered or diffracted by the alignment mark, including the LIA system and the LSA system and the like, have the same problems.
The present invention was conceived in order to overcome these problems inherent in the conventional art, and aims to provide a projection exposure apparatus that can maintain accurate alignment performance by adopting an alignment system that utilizes laser light as the alignment light.
To achieve the object mentioned above, according to the invention, a means for controlling the coherence length of the laser light is provided in the alignment system. Laser light having a short coherence length, such as a multi-mode laser to beam, is used as the alignment light. In this arrangement, unnecessary interference is eliminated, and influences on the laser light source from the temperature changes, the returning light, or the like are reduced, thereby stabilizing detection signals detected by the alignment system.
The projection exposure apparatus of the invention includes a projection exposure apparatus that has a projection optical system for projecting a pattern formed on the mask onto a photosensitive substrate, and an alignment system for aligning the mask and the photosensitive substrate. The alignment system includes a laser light source for generating laser light, and structure controlling the coherence length of the laser light. The present invention is efficient particularly when the laser light source is a semiconductor laser. According to the present invention, the coherence length of the laser light from the semiconductor laser is controlled by a high-speed switching circuit that turns the semiconductor laser oscillation on and off.
The present invention is particularly efficient when it is applied to alignment systems having a laser light source, an illumination mechanism that guides first and second laser beams emitted from the laser light source along two different paths onto a mask and/or a grid-shaped alignment mark formed on the photosensitive substrate, and a detection mechanism that detects the interference signals of the diffracted light of the first and second laser beams generated by the grid-shaped alignment mark.
Laser light generated by inductive discharge is coherent light whose waveform has a constant phase and is spatially preserved for a long period. Since ordinary light generated by natural emission, such as light emitted from an incandescent lamp, is not coherent light, it hardly interferes. Coherence length (coherence distance) is often used to represent the order of interference of the laser light. The coherence length is proportional to the square of the central wavelength and inversely proportional to the amplitude. If the coherence length is long, two light waves are likely to interfere with each other. In order to elongate the coherence length with constant wavelength, monochromatic single-mode oscillation with a small amplitude is performed. Conversely, if the coherence length is short, it becomes difficult for two light waves to interfere with each other. In order to shorten the coherence length, multi-mode oscillation with a wide range of oscillation wavelength is performed as multi-mode oscillation does not cause the mode hop phenomena and therefore, does not make the oscillation wavelength unstable.
As the driving current of the semiconductor increases, laser oscillation starts when the driving current reaches a certain level Is, as shown in FIG. 6, and light is output. In the region beyond the laser oscillation starting current Is, the light output is proportional to the excitation current of the semiconductor laser. On the other hand, the oscillation mode of the semiconductor laser has high order of dependence on the light output and changes as the light output changes as shown in FIG. 7. According to the example in FIG. 7, the semiconductor laser oscillates in the multi-mode until the light output reaches 1 mW. However, it oscillates in the single-mode when the light output becomes 5 mW. As shown in this example, the semiconductor laser usually oscillates in the single-mode and is accompanied with the mode hop phenomena, in which the laser oscillation wavelength irregularly oscillates as time passes.
Semiconductor lasers usually oscillate in the multi-mode during the first several nanoseconds after the beginning of the laser oscillation. Hence, by attaching a high-speed switching circuit to the driving circuit of the semiconductor laser and turning on and off the driving current at high speed, multi-mode oscillation is achieved, as shown in FIG. 8. Since any two consecutive modes of multi-mode oscillation have an equal frequency difference and a constant phase relation, the light level of the semiconductor laser that illuminates the alignment mark is kept constant. In addition, the coherence length, which determines the coherence between the multi-superimposed reflected/diffracted light and the 0-th order diffracted light, is shortened. As a result, unnecessary interference is eliminated, and the detection signal of the alignment system is kept stable.